What about using a single attribute `allowpresentation` not only for
default URL:
```
<iframe src="https://example.org/video/1234"
allowpresentation="none|any|default|{presentationURL}"></iframe>
```
* presentation is not allowed by default (default value for
`allowpresentation` is `none`).
* allow any presentationURL if value of `allowpresentation` is `any`.
* `allowpresentation="default"` has the same effect as
`allow-default-presentation`.
* a specific `presentationURL` can be used to specify the allowed
presentation.
--
GitHub Notif of comment by louaybassbouss
See
https://github.com/w3c/presentation-api/issues/79#issuecomment-94412353